The present invention relates to an electronic control system for controlling an internal combustion engine, and more particularly to a method of examining the operation of such an electronic control system.
Many automotive internal combustion engines are equipped with electronic engine control systems. Various different engine operation-related factors can be controlled by such an electronic control system. Major examples of such controllable factors include the ignition spark timing, the timing of fuel injection, and the amount of fuel to be injected. These controllable factors must be controlled according to the angular position of the crankshaft. To find the angular position of the crankshaft, a crankshaft sensor is required to determine the speed of rotation of the engine and the positions of the respective pistons in engine cylinders. More specifically, the crankshaft sensor operates to gain information on the phase of the engine crankshaft, which is the relative angular position of the crankshaft during one cycle of engine operation with respect to a reference crank angle position. This may be the angular position of the crankshaft when a certain piston is in the top dead center. The crankshaft phase information can be calculated from (1) information on the reference crank angle position which is obtained when the crankshaft reaches the reference angular position, and (2) information on the rotational speed of the crankshaft which is obtained by detecting the rotational speed of the crankshaft. One known means for producing such crankshaft phase information comprises a pulse signal generating mechanism including a toothed wheel of a magnetic material mounted on the engine crankshaft. An electromagnetic transducer or pickup is provided for generating an electric pulse in response to detection of each tooth of the toothed wheel as it passes by the electromagnetic pickup. One example of a reference angular position detector employing such a pulse signal generating mechanism is disclosed in Japanese Utility Model Publication No. 58(1983)-26339. The disclosed reference angular position detector includes a rotor of a magnetic material attached to the crankshaft of an engine, the rotor having teeth disposed on its outer circumferential surface at angular intervals of a unit rotational angle .alpha..degree. for generating rotational angle information. The rotor also has one tooth-free recess for producing information on a reference crank angle position. The angular position detector also includes a pair of first and second electromagnetic pickups positioned adjacent to the rotor and angularly spaced along the outer circumferential surface of the rotor by a distance corresponding to a multiple of the unit rotational angle .alpha..degree.. As the engine rotates, the first and second electromagnetic pickups produce first and second pulse signals, respectively, which are subtracted one from the other to generate a differential output. The differential output is employed to detect when the second pulse signal from the second electromagnetic pickup is not generated at the tooth-free recess and only the first pulse signal from the first electromagnetic pickup is produced. The crank angle position at the time such a condition arises is detected as the reference crank angle position.
The electronic control system includes a sensor for obtaining information, a driver for driving an actuator, and an electronic control unit (hereinafter referred to as an "ECU") for processing a signal from the sensor to apply a command to the driver. The ECU comprises a computer in most cases. If the pulse signal generating mechanism which supplies the ECU with electronic pulses generated by the electromagnetic pickup is employed as a crankshaft sensor, the pulse signal generating mechanism is susceptible to operational error since it is placed in a relatively harsh environment. There are two main causes of such operational error. First, sparks generated by ignition plugs produce strong electromagnetic waves which induce noise in wires interconnecting the pulse signal generating mechanism and the ECU, and such noise and a normal pulse signal are applied together to the ECU, tending to cause the ECU to malfunction. Secondly, signal pulses induced by the electromagnetic pickup become weaker as the rotational speed of the engine and hence the rotational speed of the toothed wheel are lowered. When the engine speed is extremely low, the pulse signal is too weak to act properly on the ECU. At any rate, inasmuch as the ECU operates on wrong information, its operation becomes inaccurate.